In recent years, mobile telephones and portable information terminal devices employ a piezoelectric vibrator using quartz crystal or the like as a time source, a timing source of control signals or the like, a reference signal source, etc. As this type of piezoelectric vibrator, various ones are offered. As one of them, a surface mount device-type piezoelectric vibrator is known. As the piezoelectric vibrator of the type, generally known is a three-layer structure type one in which a piezoelectric substrate with a piezoelectric vibration member formed thereon is sandwiched between a base substrate and a lid substrate and bonded all together. In this case, the piezoelectric vibrator is housed in the cavity (sealed unit) formed between the base substrate and the lid substrate. Recently, not only the above-mentioned three-layer structure type one but also a two-layer structure type one has been developed.
The piezoelectric vibrator of the type has a two-layer structure in which the base substrate and the lid substrate are directly bonded to each other; and a piezoelectric vibration member is housed in the cavity formed between the two substrates.
As compared with a three-layer structure one, the two-layer structure type piezoelectric vibrator is excellent in that it can be thinned, and is therefore favorably used. As one of such two-layer structure type piezoelectric vibrators, a piezoelectric vibrator is known, in which the piezoelectric vibration member is electrically connected to the external electrode formed on the base substrate using the electroconductive member formed to run through the base substrate (see Patent Reference 1 and Patent Reference 2).
The piezoelectric vibrator 600 comprises, as shown in FIG. 41 and FIG. 42, a base substrate 601 and a lid substrate 602 anodically-bonded to each other via a bonding film 607, and a piezoelectric vibration member 603 sealed up in the cavity C formed between the two substrates 601 and 602. The piezoelectric vibration member 603 is, for example, a tuning fork-type vibration member, and this is mounted on the upper face of the base substrate 601 via an electroconductive adhesive E in the cavity C.
The base substrate 601 and the lid substrate 602 are, for example, insulating substrates of ceramics, glass or the like. Of the two substrates 601 and 602, the base substrate 601 has through-holes 604 running through the substrate 601. The through-hole 604 is filled with an electroconductive member 605 to seal up the through-hole 604. The electroconductive member 605 is electrically connected to the outer electrode 606 formed on the lower face of the base substrate 601, and is electrically connected to the piezoelectric vibration member 603 mounted in the cavity C.    Patent Reference 1: JP-A 2002-124845    Patent Reference 2: JP-A 2006-279872
In the above-mentioned, two-layer structure type piezoelectric vibrator, the electroconductive member 605 plays important two roles of blocking the through-hole 604 to thereby airtightly seal up the cavity C, and electrically connecting the piezoelectric vibration member 603 to the external electrode 606. In particular, in case where the adhesion to the through-hole 604 is insufficient, then the airtight sealing inside the cavity C may be lost; and in case where the contact with the electroconductive adhesive E or the external electrode 606 is insufficient, then the piezoelectric vibration member 603 may work erroneously. Accordingly, for evading such failures, the electroconductive member 605 must be formed in such a state that it completely blocks the through-hole 604 while kept in firm contact with the inner face of the through-hole 604 and it has no depression on the surface thereof.
However, Patent Reference 1 and Patent Reference 2 describe formation of the electroconductive member 605 with an electroconductive paste (Ag paste, Au—Sn paste, etc.), but have no description relating to a concrete manufacturing method of how to practically form it.
In general, in case where an electroconductive paste is used, it must be fired and hardened. In other words, after the through-hole 604 is filled with an electroconductive paste, it must be fired and hardened. When fired, however, the organic matter in the electroconductive paste may be lost through evaporation; and therefore, in general, the volume after firing decreases as compared with that before firing (for example, in case where an Ag paste is used as the electroconductive paste, the volume may decrease by about 20% or so). Accordingly, even when the electroconductive member 605 is formed with an electroconductive paste, the surface may have depressions formed thereon or, in some serious cases, there may be a risk of forming through-holes in the center.
As a result, the cavity C may lose its airtightness, or there is a possibility that the electric connection between the piezoelectric vibration member 603 and the external electrode 606 may be lost.